In a well known method for evacuating an air space of contaminated gases, fumes, or air, it is conventional to provide a hood having a suction orifice connected to a source of negative pressure and in communication with the air space. The negative pressure or suction withdraws air from the space through the suction orifice and with the evacuation of air the contaminated gases and fumes are also withdrawn.
As illustrated in U.S. Pat. No. 4,050,367 to Eakes dated Sept. 27, 1977, it is known to direct air from a nozzle closely adjacent to and directed at the suction orifice to improve the collection of the gases into the orifice by lowering the "capture point" from the area being effected. The principal of the operation of such a device is that the Venturi effect will aid in pulling air up into the suction orifice that might normally be outside the area influenced by the suction orifice. To accomplish good results in such a situation the air nozzle should be fairly close to the suction orifice so that the air is not distributed over too wide an area and is mainly directed as the orifice.
It has become apparent that the further the "capture point" can be located from the suction orifice the more efficient the collection of exhaust gases and the less likely stray gases are to escape from the work chamber above the work table. However, if the air nozzle itself is moved too far from the suction orifice the result is likely to spread the air delivered throughout too wide an area.